Aggregates

Lower Strength with Fibers?

A participant in Aggregate Research Industries' Industry Forum (www.aggregateresearch.com/forum) recently posted an interesting question that has challenged our industry since fibers were introduced. The participant started by stating that he's noticed a disturbing trend in a recently prepared set of field cylinders. Samples batched with synthetic reinforcing fibers on one project were breaking at lower compressive strengths than mixes with comparable designs but that used different types of fibers. He said that he suspected that the contractor had authorized water additions, but field records did not support this.

He than asked, "Do you know any reasons why these strength results would be lower if fibers were used in comparable mixes?"

The question stirred interest from a wide group. Although the question seems simple, it touches on a wide range of issues of which producers should be aware when batching fiber-reinforced concrete.

Comparing the performance of a concrete mix design that is identical to others but uses a different type of fibers can become a very complex exercise. It raises at least three additional qualifying questions: What type of fiber? What percentage of mix volume do the fibers make up? What is the fibers' geometry?

Numerous types of fibers are used in concrete construction. These range metallic to mineral to synthetic. Each material can affect concrete strength in its own way. In general, it seems that most fibers should have no detrimental effect on ultimate strength. For instance, ACI 544 reports that when hardened concrete is tested, the ultimate strength is only slightly affected by the presence of steel fibers, "with observed [strength] increases ranging from 0% to 15% for up 1.5% by volume of fibers."

It seemed that the participants assumed that the query referred to concrete batched with polypropylene fibers. In the section 4.3.72 of the same ACI document, the committee generally states that "It can be said that the addition of polypropylene fibers at different quantities [ranging from 0.1-2.0] has no effect on the compressive strength." It would seem that unexpected strength loss would have to be caused by another mechanism. Many participants commented on contractor training in using fiber-reinforced concrete. One producer presented a unique observation to a similar problem.

Several years ago, the lab manager reported that the company had conducted tests that produced low compressive strengths on a project. The lab staff conducted a "root-cause analysis" and determined that the cause of the low strengths was linked to high air entrainment. Also, this problem was found in a particular brand of fiber that was used.

The lab sent samples of the fibers to two reputable laboratories for analysis. The testing lab extracted from the fibers a yellow, viscous liquid in a concentration of approximately 0.88% by weight. Using infrared spectrum analysis, the researchers determined that the liquid was an anionic surfactant, a chemical commonly used as an air-entraining additive.

The producer assumed that the surfactant was present in the fibers and/or bag as either an anti-static agent or a processing aid. But based on the significant weight of the extracted air-entraining residue from the fiber (almost 1% by weight of fibers), the fiber could contribute to the air entrainment of concrete when dosed at a rate of 1-1.5 pounds per cubic yard.

In the same sample, the second laboratory found napthalenic sulfonate derivatives (salts of petroleum acids) and the presence of methylene blue, both active surfactants which in which the presence of strong bases, e.g., lime, could facilitate a foaming reaction. Needless to say, the problem of low breaks disappeared when the producer switched to different brand of fiber.

From what most experts say, the producer's problem that developed from the packing materials is very rare. Most low-break problems are caused by perceived workability issues. It an important and complex subject that we'll discuss in a future issue.